The present invention relates to an image forming apparatus in which a plurality of developing devices are arranged around an electrostatic latent image support member.
Conventionally, a multi-color image forming apparatus is known in which a plurality of developing devices containing developers of different colors, respectively are arranged at a side of an electrostatic latent image support member. If this known multi-color image forming apparatus is used, a multi-color composite copy can be obtained through a plurality of scanning operations and changeover of the developing devices at the time of each of the scanning operations. Furthermore, as disclosed in Japanese unexamined Patent Publication No. 203474/1986, if this known multi-color image forming apparatus is used, it is also possible to obtain a so-called simultaneous color copy in which a multi-color image is formed through selective use of the developing devices during one scanning operation.
However, in the case where the above described multi-color composite copying or simultaneous color copying is performed by the known multi-color image forming apparatus, development is performed by using, for example, an upstream one of the developing devices. At this time, if developer of a downstream one of the developing devices is held in contact with a surface of the electrosratic latent image support member, such a problem arises that the image developed by the upstream developing device is disturbed by the developer of the downstream developing device.
Meanwhile, the known image forming apparatus may be of a type in which a driving force of a main motor is selectively transmitted to only a usable one of the developing devices. In this case, when changeover from drive of the upstream developing device to drive of the downstream developing device is performed during simultaneous color copying, the downstream developing device is set to a developable state immediately after the changeover, so that such an inconvenience is incurred that a rear end portion of the image forced by the upstream developing device is disturbed by the developer of the downstream developing device in the same manner as described above.